I have developed a new approach for examining the secondary and tertiary structure of single-stranded RNAs by electron microscopy. The studies to date have been performed primarily with RNA from the bacteriophage MS2. They have shown that information can be obtained about the approximate position of hairpin loops within the genetic map of the virus and about the manner in which the entire molecule folds. The studies suggest that two of the ribosome binding sites as well as the cleavage site for the endonuclease ribonuclease IV may be affected by the three dimensional folding of the molecule. In the current proposal I will attempt to prove the resolution of the studies with MS2 RNA. I also propose to begin similar studies with RNA from poliovirus. Polio virion RNA is a messenger RNA which can be isolated cleanly and in large quantity from eucaryotic cells. It provides a useful model system for the development of techniques applicable to studying the role of structural features on other viral and eucaryotic mRNAs in protein synthesis and RNA processing.